As industrial society develops toward an advanced information age, the importance of electronic displays as a medium for displaying and transferring various pieces of information is increasing day by day. Conventionally, a CRT (Cathode Ray Tube), which is bulky, was widely used therefor, but faced considerable limitations in terms of the space required to mount it, thus making it difficult to manufacture CRTs having larger sizes. Accordingly, CRTs are being replaced with various types of flat panel displays, including LCDs, plasma display panels (PDPs), field emission displays (FEDs), and organic electroluminescent displays. Among such flat panel displays, in particular, LCDs, a technologically intensive product realized from a combination of liquid crystal-semiconductor techniques, are advantageous because they are slim and lightweight and consume little power. Therefore, research and development into structures and manufacturing techniques thereof is continuing. Nowadays, LCDs are already applied in fields such as notebook computers, monitors for desktop computers, and portable personal communication devices (including PDAs and mobile phones). Besides, LCDs are being applied to large-sized TVs, such as HD (High-Definition) TVs, as technology to enlarge their size is overcoming its limitations. Thereby, LCDs are receiving attention as novel displays able to substitute for CRTs, which used to be synonymous for displays.
In such an LCD, a backlight unit, which is a light source device, is an important part determining the properties of the LCD, which is an indirect light-emitting type, showing an image by controlling the transmittance of an external light source.
In particular, as a technique for fabricating an LCD panel is developed, the demand for LCDs which are slim and highly luminant is increased. Accordingly, various attempts to increase the luminance of the backlight unit have been made. The LCD suitable for use in monitors, PDAs (Personal Digital Assistants), and notebook computers is evaluated to be superior when it emits bright light rays from a low energy source. Thus, in the case of the LCD, front-surface luminance is considered very important.
Because the LCD has a structure in which light passing through a light diffusion layer is diffused in all directions, the amount of light traveling toward the front surface becomes very insufficient. Hence, great effort is continually made to exhibit higher luminance with lower power consumption. Further, as the area of the display is enlarged, the viewing angle is required to be wider to enable the image to be viewed by more observers.
To this end, the power of the backlight unit is increased. However, power consumption and concomitant power loss attributable to heat are increased in proportion thereto. In the case of portable displays, the capacity of the battery must be increased, and the life span thereof is shortened.
Hence, with the goal of increasing the luminance, a sheet having a structured layer on which a three-dimensional (3D) structured surface is formed is layered on a light diffusion sheet to impart directionality to light. Such a sheet is illustrated in a manner that includes a transparent resin-cured layer, having a structured surface on one surface thereof and a smooth flat surface on the other surface thereof, and a substrate layer formed on the smooth flat surface of the resin-cured layer to be in contact therewith. The structured layer typically has an array of columns having a triangular cross-section, or alternatively, various forms may be provided.
Because the structured surface generally has individual structures having the same form which are linearly arranged, there is a large concern about damage to the peaks of the structured surface. Further, the angles emitted from such structures are the same in the array, and thus, a difference in the emitted light path between the damaged portion and the peak occurs even when the peaks of the structures are slightly destroyed or small scratches are created on the tilted surface thereof, consequently decreasing the luminance thereof and increasing defective rates.
So, in the production of an optical sheet having a 3D structured surface, problems in which the front surface of the produced sheet is not used depending on the position of small defects may arise, undesirably leading to a decrease in productivity and thus to a burden of high cost. Actually, manufacturers who assemble backlight modules suffer in that, when such a sheet is handled, defective rates attributable to damage to the structures by scratches are considerably problematic. In particular, the optical sheet having a 3D structured surface is disadvantageous in that the peaks of the structured surface may be easily destroyed or damaged even by small external scratches.
Furthermore, in the backlight unit consisting of a plurality of sheets and films, which are layered, when a plurality of optical sheets having 3D structured surfaces is provided to increase luminance, damage to 3D structures must be prevented. This is considered very important.
Hence, in order to prevent damage to the structures, conventional cases in which a protective film is provided have been proposed. However, as the LCD panel becomes slimmer, the general trend is to omit such a film or to use a sheet having a composite function, and also, if a process for forming a protective film is added, the production cost is increased and temporal and physical efficiencies are decreased.
In addition to damage to the structures attributable to handling, while portable displays, such as notebook computers and PDAs, are increasingly used, they are frequently transported in a state of being placed in a bag. During the transport thereof, when impact is applied to the display when a user runs or a car stops suddenly, the structure of the optical sheet in the display is damaged even in the presence of the protective film, negatively affecting the image screen.
Therefore, an optical sheet having a structured surface, capable of flexibly accommodating external impact, is urgently required.